We recently generated the first viable mouse model of XNDI in which the V2R gene can be deleted in a conditional (4-OH-tamoxifen-inducible) fashion in the kidneys of adult mice. The resulting V2R KO mice showed all key symptoms of XNDI, including the production of large amounts of dilute urine (polyuria) and polydipsia (Li JH et al. J. Clin. Invest. 119, 3115-3126, 2009). To identify drugs useful for the treatment of XNDI, we decided to take a comprehensive and systematic approach. The synthesis and function of the water channel that allows water reuptake in kidney collecting ducts (aquaporin 2) requires that the cAMP levels in the renal collecting duct cells reach a certain level. However, in XNDI mice, cAMP levels are low due to the absence of V2Rs. Moreover, due to low cAMP levels, the amount of aquaporin 2 protein expressed by renal collecting duct cells is dramatically reduced (Li JH et al. J. Clin. Invest. 119, 3115-3126, 2009). We therefore reasoned that agents that can stimulate cAMP levels in renal collecting duct cells may have considerable potential for the treatment of XNDI. To greatly increase the likelihood to identify suitable XNDI drugs, we started to collaborate with the NIH National Genomics Center (NGC) to carry out screens of large drug libraries. In collaboration with the NGC, we carried out screens using cultured renal cells that have similar properties as renal collecting duct cells (mpkCCD cells). These screens can identify compounds that are able to raise cAMP levels in these cells. So far, the following libraries have been screened for compounds that can increase cAMP levels in mpkCCD cells: 1. LOPAC library (Library of Pharmacologically Active Compounds) This is a compound library from Sigma-Aldrich with 1280 pharmacologically known compounds. 2. FDA library This is a library of all compounds approved by the FDA as well as compounds approved in Europe and Japan (about 3800 pharmacologically active compounds). This screening effort has identified many compounds that can increase cAMP levels in mpkCCD cells with high potency and efficacy. Before testing these compounds in the XNDI mice, we are currently in the process of confirming that these drugs are also active in collecting duct tubules freshly prepared form mouse kidneys. This is a critical step since it is not fully clear to which extent mpkCCD cells grown in culture mimic actual collecting duct cells. We are also setting up a system that will allow the screening of the above libraries with freshly prepared collecting duct tubules. These tubules will be prepared from RAT kidneys since the amount of tissue that can be obtained from the mouse is very limited. Once active compounds have been identified in these in vitro screens, these drugs will be tested in XNDI mice.